When daylight time ends, an hour repeats on the local clock (for example,
in US Eastern, the clock jumps from 1:59 back to 1:00 again).  Times in
the repeated hour are ambiguous.  A tzinfo subclass that wants to play
with astimezone() needs to treat times in the repeated hour as being
standard time.  astimezone() previously required that such times be
treated as daylight time.  There seems no killer argument either way,
but Guido wants the standard-time version, and it does seem easier the
new way to code both American (local-time based) and European (UTC-based)
switch rules, and the astimezone() implementation is simpler.